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Bloated doughnut stars?


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Evening folks,

I was imaging M57 last night and i've noticed two irritating things, the first is i occasionally get stars appearing double as if there scope has slipped even though i'm guiding in PHD and after processing in DSS and removing these frames I appear to be getting these odd doughnuts and faded stars.

The stacked image is made from 20x 180s lights and 10x darks at 1600iso. Any help and advice would be appreciated.

Thanks

post-12650-0-53021000-1367961020_thumb.j

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It looks like focus to me too. Could you post a single sub? Have you checked every sub to see if it isn't just one bad one?

Double star-like images are usually the result of backlash. The scope can oscillate across the free play in the worm gears. The usual fix is to balance the scope a little out and heavy on the east side, so the gears are always pushing against the payload. Whether that is weights heavy or scope side heavy depends on the orientation of the scope, but in the N hemisphere it is always east side heavy. Not too much, just a tad.

Olly

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It looks like focus to me too. Could you post a single sub? Have you checked every sub to see if it isn't just one bad one?

Double star-like images are usually the result of backlash. The scope can oscillate across the free play in the worm gears. The usual fix is to balance the scope a little out and heavy on the east side, so the gears are always pushing against the payload. Whether that is weights heavy or scope side heavy depends on the orientation of the scope, but in the N hemisphere it is always east side heavy. Not too much, just a tad.

Olly

Browsing the forums really does throw up some gems.

Thanks for that Olly, I'll try that myself. Will that affect the smoothness of the PHD graph? I'm already a bit wobbly as I haven't sorted out PEC yet, but I'm nervous about balance as it is....

Posted via Tapatalk on an ageing iPhone so please excuse any erroneous spellings or accidental profanities!

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Browsing the forums really does throw up some gems.

Thanks for that Olly, I'll try that myself. Will that affect the smoothness of the PHD graph? I'm already a bit wobbly as I haven't sorted out PEC yet, but I'm nervous about balance as it is....

Posted via Tapatalk on an ageing iPhone so please excuse any erroneous spellings or accidental profanities!

Yes, it should improve it. Whether or not to apply PEC and autoguiding is a moot point. I never have done so but others train the PEC with an autoguider and then run both. I think you need to try it. Balance is the biggest single factor, along with having reasonable PA, but a small PA error is something that some people go for deliberately. The idea is that the guide corrections will all go the same way in Dec so you can de-activate corrections in one direction. This can reduce a tenedency to bounce back and forth across the right position. I have never tried this systematically, I must admit.

Olly

Edit; Oh, another thing is the duration of your subs. As long as the PE of the mount will allow is the best. The Mesu thrives on 5 seconds and these long exposures average out the seeing so you don't chase it. More twitchy mounts need a slapped wrist more often so you have to go as low as half a second sometimes, but try to make it 2 seconds at least and only come below that if you really find you have to.

Edited by ollypenrice
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Yes I did see that PEC may or may not help when auto guiding, which leads me to wonder: the absolute best guiding I can wrestle from my settings is still resulting in some error. The graph is nice and smooth for about 30s but then proceeds to oscillate for around a minute. It then settles and the cycle repeats. If this isn't to do with periodic error, is it more likely PA or balance?

Posted via Tapatalk on an ageing iPhone so please excuse any erroneous spellings or accidental profanities!

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Thanks for the feedback folks, the design hot cosmetic pixel removal sounds a likely culprit as the focus on the sub is good. Thanks backlash remedy from Olly also sounds promising, thanks.

I'll post a single sub when I get home, I'm on the Isle of Skye right now, hopefully get a bit of time lapse in if I can stay up long enough for some proper darkness.

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